Waveguide systems for high microwave powers of for example several 100 kW per waveguide at frequencies up to substantially above 300 MHz are required for example in plasma physics, fusion reactors, particle accelerators and the like. At frequencies of about 100 GHz the waveguides already have relatively small diameters so that at high microwave powers in the interior of the waveguides very high electrical field strengths occur. To cope with such high electrical field strengths it is known to fill waveguides for microwaves with a gas of good insulating properties. The insulating gas filling may be pressurized. Nevertheless, the high field strengths still lead occasionally to internal arcing. When such arcing occurs gaseous compounds result which impair the dielectric strength of the gas in the interior of the waveguide. To avoid this undesirable effect of interior arcing the insulating gas disposed in the interior of the waveguide must be continuously replaced.
Austrian patent specification No. 228,843 discloses a cavity resonator whose casing consists of at least one ferrite ring whose inner surface is coated with a thin silver layer. Ferrites are admittedly made generally by a sintering technique but they are not porous.
German patent specification No. 892,150 discloses a waveguide system (cavity resonator, hollow waveguide) whose housing is internally lined with a sort of plaited high-frequency litz. Even if this lining were permeable to gas, gas exchange between the interior and exterior of the cavity would be prevented by the impermeable housing.